Implantable stimulation devices generate and deliver electrical stimuli to nerves and tissues for the therapy of various biological disorders, such as pacemakers to treat cardiac arrhythmia, defibrillators to treat cardiac fibrillation, cochlear stimulators to treat deafness, retinal stimulators to treat blindness, muscle stimulators to produce coordinated limb movement, spinal cord stimulators to treat chronic pain, cortical and deep brain stimulators to treat motor and psychological disorders, occipital nerve stimulators to treat migraine headaches, and other neural stimulators to treat urinary incontinence, sleep apnea, shoulder subluxation, etc. The present invention may find applicability in all such applications and in other implantable medical device systems, although the description that follows will generally focus on the use of the invention in a Bion® microstimulator device system of the type disclosed in U.S. Patent Application Publication 2010/0268309.
Microstimulator devices typically comprise a small, generally-cylindrical housing which carries electrodes for producing a desired stimulation current. Devices of this type are implanted proximate to the target tissue to allow the stimulation current to stimulate the target tissue to provide therapy for a wide variety of conditions and disorders. A microstimulator usually includes or carries stimulating electrodes intended to contact the patient's tissue, but may also have electrodes coupled to the body of the device via a lead or leads. A microstimulator may have two or more electrodes. Microstimulators benefit from simplicity. Because of their small size, the microstimulator can be directly implanted at a site requiring patient therapy.
FIG. 1 illustrates an exemplary implantable microstimulator 100. As shown, the microstimulator 100 includes a power source 145 such as a battery, a programmable memory 146, electrical circuitry 144, and a coil 147. These components are housed within a capsule 202, which is usually a thin, elongated cylinder, but may also be any other shape as determined by the structure of the desired target tissue, the method of implantation, the size and location of the power source 145, and/or the number and arrangement of external electrodes 142. In some embodiments, the volume of the capsule 202 is substantially equal to or less than three cubic centimeters.
The battery 145 supplies power to the various components within the microstimulator 100, such as the electrical circuitry 144 and the coil 147. The battery 145 also provides power for therapeutic stimulation current sourced or sunk from the electrodes 142. The power source 145 may be a primary battery, a rechargeable battery, a capacitor, or any other suitable power source. Systems and methods for charging a rechargeable battery 145 will be described further below.
The coil 147 is configured to receive and/or emit a magnetic field that is used to communicate with, or receive power from, one or more external devices that support the implanted microstimulator 100, examples of which will be described below. Such communication and/or power transfer may be transcutaneous as is well known.
The programmable memory 146 is used at least in part for storing one or more sets of data, including electrical stimulation parameters that are safe and efficacious for a particular medical condition and/or for a particular patient. Electrical stimulation parameters control various parameters of the stimulation current applied to a target tissue including the frequency, pulse width, amplitude, burst pattern (e.g., burst on time and burst off time), duty cycle or burst repeat interval, ramp on time and ramp off time of the stimulation current, etc.
The illustrated microstimulator 100 includes electrodes 142-1 and 142-2 on the exterior of the capsule 202. The electrodes 142 may be disposed at either end of the capsule 202 as illustrated, or placed along the length of the capsule. There may also be more than two electrodes arranged in an array along the length of the capsule. One of the electrodes 142 may be designated as a stimulating electrode, with the other acting as an indifferent electrode (reference node) used to complete a stimulation circuit, producing monopolar stimulation. Or, one electrode may act as a cathode while the other acts as an anode, producing bipolar stimulation. Electrodes 142 may alternatively be located at the ends of short, flexible leads. The use of such leads permits, among other things, electrical stimulation to be directed to targeted tissue(s) a short distance from the surgical fixation of the bulk of the device 100.
The electrical circuitry 144 produces the electrical stimulation pulses that are delivered to the target nerve via the electrodes 142. The electrical circuitry 144 may include one or more microprocessors or microcontrollers configured to decode stimulation parameters from memory 146 and generate the corresponding stimulation pulses. The electrical circuitry 144 will generally also include other circuitry such as the current source circuitry, the transmission and receiver circuitry coupled to coil 147, electrode output capacitors, etc.
The external surfaces of the microstimulator 100 are preferably composed of biocompatible materials. For example, the capsule 202 may be made of glass, ceramic, metal, or any other material that provides a hermetic package that excludes water but permits passage of the magnetic fields used to transmit data and/or power. The electrodes 142 may be made of a noble or refractory metal or compound, such as platinum, iridium, tantalum, titanium, titanium nitride, niobium or alloys of any of these, to avoid corrosion or electrolysis which could damage the surrounding tissues and the device.
The microstimulator 100 may also include one or more infusion outlets 201, which facilitate the infusion of one or more drugs into the target tissue. Alternatively, catheters may be coupled to the infusion outlets 201 to deliver the drug therapy to target tissue some distance from the body of the microstimulator 100. If the microstimulator 100 is configured to provide a drug stimulation using infusion outlets 201, the microstimulator 100 may also include a pump 149 that is configured to store and dispense the one or more drugs.
Turning to FIG. 2, the microstimulator 100 is illustrated as implanted in a patient 150, and further shown are various external components that may be used to support the implanted microstimulator 100. An external controller 155 may be used to program and test the microstimulator 100 via communication link 156. Such link 156 is generally a two-way link, such that the microstimulator 100 can report its status or various other parameters to the external controller 155. Communication on link 156 may occur, e.g., via magnetic inductive coupling. Thus, when data is to be sent from the external controller 155 to the microstimulator 100, a coil 158 in the external controller 155 is excited to produce a magnetic field that comprises the link 156, which magnetic field is detected at the coil 147 in the microstimulator. Likewise, when data is to be sent from the microstimulator 100 to the external controller 155, the coil 147 is excited to produce a magnetic field that comprises the link 156, which magnetic field is detected at the coil 158 in the external controller. Typically, the magnetic field is modulated, for example with Frequency Shift Keying (FSK) modulation or the like, to encode the data. The external controller 155 is typically sized to be a hand-holdable device containing a user interface for controlling and monitoring its operation, as is well known in the art.
An external charger 151 provides power used to recharge the battery 145 (FIG. 1). Such power transfer occurs by energizing the coil 157 in the external charger 151, which produces a magnetic field comprising link 152. This magnetic field 152 energizes the coil 147 through the patient 150's tissue, and which is rectified, filtered, and used to recharge the battery 145 as explained further below. Link 152, like link 156, can be bidirectional to allow the microstimulator 100 to report status information back to the external charger 151. For example, once the circuitry 144 in the microstimulator 100 detects that the power source 145 is fully charged, the coil 147 can signal that fact back to the external charger 151 so that charging can cease. Charging can occur at convenient intervals for the patient 150, such as every night. Like the external controller 155, the external charger 151 is typically a hand held device containing a user interface for controlling and monitoring its operation, again as is well known in the art.
FIGS. 3A and 3B illustrate salient portions of the microstimulator's power circuitry 160. When the coil 157 in the external charger 151 is stimulated by AC current Iprim, a magnetic charging field 161 is produced. This field 161 (comprising part of link 152) is received at coil 147 in the microstimulator 100. The coil 147 in combination with capacitor 162 comprises a resonant circuit, or tank circuit, which produces an AC voltage at Va. This AC voltage is rectified by rectifier circuitry 164, which can comprise a well-known four-diode bridge circuit, although it is shown in FIG. 3B as a single diode for simplicity. Capacitor 166 assists to filter the signal at node Vb, such that Vb is essentially a DC voltage, although perhaps having a negligible ripple. Intervening between Vb and the rechargeable battery 145 is charging circuitry 170, which ultimately takes the DC voltage Vb and uses it to produce a controlled battery charging current, Ibat. Charging circuitry 170 is well known. One skilled in the art will recognize that the power circuitry 160 may include other components not shown for simplicity.
Also shown in FIG. 3B is a parameter called Vnab. Vnab comprises a voltage in the power circuitry 160 within the implant 100, and in particular comprises a voltage drop across the charging circuitry 170 when the power circuitry 160 is receiving a magnetic charging field. Vnab is computed as the difference between the DC rectified voltage, Vb, and the battery voltage, Vbat, i.e., Vnab=Vb−Vbat. As explained in U.S. Patent Application Publication 2011/0121777 (“the '777 Publication”), which is incorporated herein by reference in its entirety, Vnab scales with the power received from the external charger. Because the degree of coupling will affect the receipt of such power, Vnab is indicative of the coupling. As such, Vnab can comprise (or can be used to derive) a coupling parameter between the external charger 151 and the microstimulator 100. Note that Vnab takes into account all factors affecting coupling, including distance, offset, and angle between the coils in the microstimulator and the external charger.
Depending on the patient's condition, it may be desirable to implant more than one microstimulator to provide more complex stimulation to the patient and/or to provide stimulation in different locations. For instance, as shown in FIG. 4, a first microstimulator 1001 is implanted at a first location, and a second microstimulator 1002 is implanted at a second location. Additional microstimulators could also be implanted if more complicated therapies are indicated, but only two microstimulators are shown in FIG. 4 for simplicity. Microstimulators 1001 and 1002 may operate independently or may operate in a coordinated manner.
The external controller 155 can communicate with each microstimulator independently, with communications accompanied by a header containing an address of the microstimulator. Such addressing ensures no confusion when communicating with the two microstimulators 1001 and 1002, and thus allows each to be independently programmed and monitored by the external controller 155. Such addressing also allows the two microstimulators 1001 and 1002 to communicate with each other.
Both microstimulators 1001 and 1002 will eventually need to have their batteries recharged using external charger 151, and such charging presents special challenges. Each of the microstimulators 1001 and 1002 could be charged independently, but this would take additional time. Even if a patient had only two microstimulators implanted, the total time to charge both would roughly double compared to a single implant, which would comprise a major inconvenience to the patient. Independent charging of the microstimulators also requires some coordination between the microstimulators 1001 and 1002. For example, the microstimulators 1001 and 1002 would have to know when to enable or disable charging by opening or connecting their coils 147.
Because of such issues, the inventors consider it preferable to charge both microstimulators 1001 and 1002 at the same time. However, while this approach would provide for faster charging, it is a challenge to optimize and to do so safely. Of particular concern is implant heating, which one skilled in the art will understand is an inevitable side effect of charging using magnetic fields. Heating can result from several different sources, such as eddy currents in conductive portions of the implant, or heating of the various components in the power circuitry 160. Implant heating is a serious safety concern; if an implant exceeds a given safe temperature (e.g., 41° C.), the tissue surrounding the implant may be aggravated or damaged.
Generally speaking, implant heating is a function of both the strength of the magnetic charging field, and the coupling between the external charger 151 and the implant. The strength of the magnetic charging field can be increased by increasing the excitation current, Iprim, in the coil 157 of the external charger 151 (FIGS. 3A and 3B). Increasing the magnetic charging field will increase the current/voltage induced in the coil 147 of the microstimulator 100, which increases the battery charging current, Ibat (FIG. 3B). Increasing the battery charging current speeds up charging, but also increases heat dissipation in the device.
Coupling between the external charger 151 and the implant affects how readily the magnetic charging field is passed to the implant, i.e., how strongly the effect of the magnetic charging field is felt at the implant. Many factors affecting coupling, such as the inductances of the coil 157 in the external charger 151 and the coil 147 in the implant; alignment, angle and distance between the coils 151 and 147; the permittivity of any materials (e.g., tissue, air) between the coils, etc. Generally speaking, if the coupling between the coils is relatively high, a relatively large current/voltage will be induced in implant coil 147, leading to faster charging and higher power dissipation (higher temperatures) in the implant.
Because of differences in the placement of multiple microstimulators in a patient, one could expect that the coupling between the external charger 151 and each of those microstimulators would differ. This means that the same magnetic charging field produced by the external charger 151 would result in different amounts of power dissipation in each of the microstimulators. Consider FIG. 4: microstimulator 1002 is located deeper in the patient, and is therefore farther away from the external charger 151 than is microstimulator 1001. Moreover, the angle θ between the coil 147 in microstimulator 1002 and coil 157 in external charger 151 is relatively large, and the offset of their axes D is relatively large. These factors all contribute to low coupling between the external charger 157 and microstimulator 1002 as compared to microstimulator 1001.
As a result, when the external charger 151 produces a magnetic charging field, microstimulator 1001 will charge more quickly—and will generate more heat—than will microstimulator 1002. As noted, this makes optimization difficult. If the generated magnetic charging field is optimized to charge microstimulator 1002 as quickly as possible at a safe temperature, then microstimulator 1001 would become too hot. By contrast, if the generated magnetic charging field is optimized to charge microstimulator 1001 as quickly as possible at a safe temperature, then microstimulator 1002 would charge too slowly.